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Abstract:

A device for compensating for the frequency of a resonator, includes: a
temperature sensor for the resonator; a sequencer determining a second
compensation signal on the basis of the temperature value corresponding
to a positive value N, and a third compensation signal on the basis of
the temperature value, corresponding to a ratio between a positive
integer S and N, S being lower than or equal to N; a variable counter
receiving the compensation signals and generating a fourth output signal
every N periods of a clock signal from the resonator and generating a
fifth signal for modifying the charge capacity of the resonator. N
includes an integer part Nint and a fractional part Nfrac, and the
variable counter includes: an accumulator receiving Nfrac; a dual-module
Nint, Nint+1 counter receiving Nint, a counting member connected to the
output of the sequencer and to the output of the dual-module counter, and
receiving the third and fourth signals, the counting element generating
the fifth signal based on the state of S.

Claims:

1-14. (canceled)

15. A device capable of compensating for the manufacturing tolerances and
the temperature drift of a reference frequency of a silicon-type
resonator, generating a programmable compensated reference frequency
Fref, comprising:a temperature sensor capable of measuring an operating
temperature of said resonator and capable of providing a first signal
corresponding to the temperature,a sequencer connected to said
temperature sensor, said sequencer being capable of determining a second
compensation signal, from the first signal and as a function of said
temperature value, using calibration data intended to be stored in a
storage means connected to said sequencer, said second signal
corresponding to a positive value N, said sequencer being, moreover,
capable of determining a third compensation signal, from said first
signal and as a function of the temperature value, using calibration
data, said third signal corresponding to a ratio between a positive
integer S and N, S being less than or equal to N,a variable counter
connected to said sequencer and connected to an oscillator provided with
said resonator, said variable counter--being capable of receiving said
compensation signals, capable of producing a fourth output signal every N
periods of a clock signal coming from said resonator, and capable of
detecting the state S, arranged to produce a fifth signal capable of
modifying the load capacity of said oscillator,wherein N comprises an
integer part Nint and a fractional part Nfrac, and in that the variable
counter comprises:an accumulator connected to the outlet of the sequencer
and capable of receiving a sixth signal corresponding to the fractional
part of N,a Nint, Nint+1 dual-module counter connected to the outlet of
the accumulator and to the outlet of the sequencer, capable of receiving
a seventh signal corresponding to the integer part of N, and whereof the
output is the compensated reference frequency Fref,a counting element
connected to the outlet of the sequencer and to the outlet of the
dual-module counter, capable of receiving said third signal and said
fourth signal, said counting element being capable of producing the fifth
signal as a function of the state S.

16. The device according to claim 15, wherein the oscillator is connected
to a bank of two capacities each respectively connected to a transistor.

17. The device according to claim 15, wherein said device comprises an
analog digital converter placed between said sensor and said sequencer
and capable of converting said operating temperature into a digital
voltage.

18. The device according to claim 16, wherein said device comprises an
analog digital converter placed between said sensor and said sequencer
and capable of converting said operating temperature into a digital
voltage.

19. The device according to claim 15, wherein said sequencer is capable of
starting said temperature sensor and capable of reading the signal
corresponding to said temperature.

20. The device according to claim 16, wherein said sequencer is capable of
starting said temperature sensor and capable of reading the signal
corresponding to said temperature.

21. The device according to claim 17, wherein said sequencer is capable of
starting said temperature sensor and capable of reading the signal
corresponding to said temperature.

22. The device according to claim 18, wherein said sequencer is capable of
starting said temperature sensor and capable of reading the signal
corresponding to said temperature.

23. The device according to claim 15, wherein said sensor and said
resonator are integrated on a same silicon substrate.

24. The device according to claim 16, wherein said sensor and said
resonator are integrated on a same silicon substrate.

25. The device according to claim 17, wherein said sensor and said
resonator are integrated on a same silicon substrate.

26. The device according to claim 19, wherein said sensor and said
resonator are integrated on a same silicon substrate.

27. The device according to claim 15, wherein said sensor and said
resonator are each respectively integrated on a silicon substrate, said
two substrates being connected by "flip-chip."

28. A method for compensating for manufacturing tolerances and for the
temperature drift of a reference frequency of a silicon-type
piezoelectric resonator comprising the following steps:turning on of a
temperature sensor by a sequencer connected to said sensor,reading a
first signal corresponding to an operating temperature measurement of
said resonator, said first signal coming from the temperature
sensor,determination by the sequencer of a second compensation signal
from said first signal using calibration data intended to be stored in a
storage means connected to said sequencer, the second signal
corresponding to a positive value N, N comprising an integer part Nint
and a fractional part Nfrac,determination by the sequencer of a third
compensation signal from said first signal using calibration data, the
third signal corresponding to a ratio between a positive integer S and N,
S being less than or equal to N,reception of the compensated signals by a
variable counter connected to the sequencer and to an oscillator provided
with said resonator, the variable counter comprising:an accumulator
connected to the output of the sequencer and capable of receiving a sixth
signal corresponding to the fractional part of N,a Nint, Nint+1
dual-module counter connected to the outlet of the accumulator and to the
outlet of the sequencer, capable of receiving a seventh signal
corresponding to the integer part of N,a counting element connected to
the outlet of the sequencer and to the outlet of the dual-module counter,
capable of receiving said third signal and said fourth signal, said fixed
counter being capable of detecting the state S and producing the fifth
signal as a function of the state S.production of a fourth output signal
by the variable counter every N periods of a clock signal coming from
said resonator,detection of the state S less than or equal to N, with the
aim of producing a fifth signal capable of modifying the load capacity of
said oscillator.

Description:

TECHNICAL FIELD

[0001]The present invention relates to the field of heating compensation
devices for piezoelectric or electrostatic resonators arranged in
oscillators.

[0002]It more particularly concerns an electronic device making it
possible to compensate for the thermal drift of the frequency of a
silicon piezoelectric or electrostatic resonator with the aim of ensuring
the stability of its reference frequency.

BACKGROUND OF THE INVENTION

[0003]The article by D. Lanfranchi et al., "A Microprocessor-Based Analog
Wristwatch Chip with 3 Seconds/Year Accuracy", IEEE 1994, describes a
circuit making it possible to increase the stability of the nominal
frequency of a quartz-type resonator typically having a frequency equal
to 32 kHz. An oscillator is connected to this resonator. A dual-frequency
operating mode of the resonator is provided: the frequency adjustment is
done by proceeding with a switching of a capacity added to the structure
of the oscillator, by an external reference signal. Thus the oscillator,
qualified as dual-frequency, can oscillate at two different frequencies:
a first frequency greater than the nominal frequency and a second
frequency lower than the nominal frequency. A ratio can be defined
between the respective average times that the oscillator spends at the
two high and low frequencies, corresponding to the first higher frequency
and the second lower frequency, respectively. This ratio is adjusted
using a reference signal. This reference signal can derive either from a
more precise second resonator or from a temperature sensor. The more
precise resonator can be used intermittently. However, it consumes too
much to play the role of a time base serving to maintain a real time
clock (RTC). Furthermore, the use of two resonators is not compatible
with a concern for maximum miniaturization of the electronic devices.

[0004]It may therefore be preferable to obtain the time base by
integrating a silicon-type low frequency resonator, combined with a
temperature sensor. For example, document JP 58 173488 discloses a
thermal compensating device for a dual-frequency oscillator, implementing
a counter allowing a periodic modification of the frequency of the
resonator as a function of the measured temperature, and therefore a
temperature compensation of the output frequency.

[0005]However, a so-called dual-frequency mode device only makes it
possible to compensate the variations of the resonator's frequency as a
function of the temperature on a scale corresponding to the so-called
draw difference between the high frequency and the low frequency. The
variation of the frequency of a silicon resonator, as a function of the
temperature, is close to 30 parts per million (ppm) per degree Celsius.
It has been shown that one could obtain a draw in the vicinity of 100 ppm
for a resonator of this type. The silicon resonator can then only be
compensated over a range of about 3 degrees Celsius, which is
insufficient for an industrial application.

[0006]In document EP 1 475 885, a thermal compensation similar to that
proposed by document JP 58 173488 is done but, furthermore, a variable
frequency divider is placed on the outlet of the oscillator, the measured
temperature acting, via a compensating circuit, on a bank of switchable
capacities controlled digitally (or on a variable capacity controlled by
a digital-analog converter) and also on the division factor of the
divider. Thus, the temperature compensation range is extended. However,
the variable frequency divider is realized using a PLL, the N/M factor of
which is close to the unit, which penalizes the consumption. Moreover,
due to the non-linearity of the adjustment features, a calibration must
be done at several different temperature points, making the device
complex to implement.

[0007]The present invention proposes a temperature compensating device for
a resonator making it possible to avoid these drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

[0008]One object of the present invention is to provide a device making it
possible to generate a programmable compensated frequency stable over an
extended temperature range from a silicon-type piezoelectric resonator.

[0009]Another object of the present invention is to provide a method for
electronic thermal compensation of the reference frequency of a
silicon-type piezoelectric resonator.

[0010]More particularly, the invention concerns a device capable of
compensating the manufacturing tolerances and the temperature drift of a
reference frequency of a silicon-type piezoelectric resonator, generating
a programmable compensated reference frequency Fref. The device
comprises: [0011]a temperature sensor capable of measuring an operating
temperature of the resonator and capable of providing a first signal
corresponding to the temperature, [0012]a sequencer connected to the
temperature sensor, the sequencer being capable of determining a second
compensation signal, from the first signal and as a function of the
temperature value, using calibration data intended to be stored in a
storage means connected to the sequencer, the second signal corresponding
to a positive value N, the sequencer being, moreover, capable of
determining a third compensation signal, from the first signal and as a
function of the temperature value, using calibration data intended to be
stored in the storage means connected to the sequencer, the third signal
corresponding to a ratio between a positive integer S and N, S being less
than or equal to N, [0013]a variable counter connected to said sequencer
and connected to an oscillator provided with said resonator, the variable
counter being capable of receiving said compensation signals, capable of
producing a fourth output signal every N periods of a clock signal coming
from said resonator, and capable of detecting the state S, arranged to
produce a fifth signal capable of modifying the load capacity of said
oscillator.

[0014]According to the invention, N comprises an integer part Nint and a
fractional part Nfrac, and the variable counter comprises: [0015]an
accumulator connected to the outlet of the sequencer and capable of
receiving a sixth signal corresponding to the fractional part of N,
[0016]a Nint, Nint+1 dual-module counter connected to the outlet of the
accumulator and to the outlet of the sequencer, capable of receiving a
seventh signal corresponding to the integer part of N, and the outlet of
which is the compensated reference frequency Fref, [0017]a counting
element connected to the outlet of the sequencer and to the outlet of the
dual-module counter, capable of receiving said third signal and said
fourth signal, said counting element being capable of producing the fifth
signal as a function of the state S.

[0018]The invention also concerns a method for compensating manufacturing
tolerances and the temperature drift of a reference frequency of a
silicon-type piezoelectric resonator comprising the following steps:
[0019]turning on of a temperature sensor by a sequencer connected to said
sensor, [0020]reading a first signal corresponding to an operating
temperature measurement of the resonator, the first signal coming from
the temperature sensor, [0021]determination by the sequencer of a second
compensation signal from the first signal using calibration data stored
in a storage means connected to the sequencer, the second signal
corresponding to a positive value N, [0022]determination by the sequencer
of a third compensation signal from the first signal using calibration
data, the third signal corresponding to a ratio between a positive
integer S and N, S being less than or equal to N, [0023]reception of the
compensated signals by a variable counter connected to the sequencer and
to an oscillator provided with the resonator, [0024]production of a
fourth output signal by the variable counter every N periods of a clock
signal coming from the resonator, [0025]detection of the state S less
than or equal to N, with the aim or producing a fifth signal capable of
modifying the load capacity of said oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]Other features of the present invention will appear more clearly
upon reading the following description, done in reference to the appended
drawing, in which:

[0027]FIG. 1 shows an oscillator structure traditionally used in
connection with a resonator;

[0028]FIG. 2 shows a compensation device of a first embodiment according
to the present invention;

[0029]FIG. 3 shows a compensation device of a second embodiment according
to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030]Out of a concern for clarity, same elements have been designated by
the same references in the different figures.

[0031]FIG. 1 shows an example of an oscillator 10 connected to a
piezoelectric resonator 11. The oscillator 10 comprises a first reverser
12 placed between the nodes of the resonator 11 and a polarization
resistance 13 connected to the terminals of the oscillator 10. In the
framework of a dual-frequency mode, the oscillator is connected to a bank
of one or two switchable capacities 15: each of the two capacities 15 is
connected to the mass by an interrupter than can be a MOS-type transistor
17. If the oscillator 10 is used as an outlet of a time base to maintain
a real time clock, a second reverser 18 identical to the first reverser
12 is connected to said oscillator.

[0032]The oscillation frequency of the oscillator 10 can be made variable
by adjusting the load capacity using the switchable capacities 15. In
dual-frequency mode, we talk about draw difference. The frequency offset
between the resonance frequency and the oscillation frequency of the
resonator 11 corresponds to this draw. For example, the oscillation
frequency depending (inversely proportionate) on the load capacity, the
draw is therefore null for an infinite load capacity. One therefore
understands that by acting on these load capacities, it is possible to
modify the oscillation frequency so as to compensate a possible thermal
drift.

[0033]FIG. 2 shows, according to a first embodiment of the invention, a
device 20 capable of compensating the temperature drift of a reference
frequency of a silicon-type piezoelectric resonator 27. It comprises a
temperature sensor 21 connected to a sequencer 22 that can be a state
machine or a microcontroller. The temperature sensor 21 can be a
transducer capable of converting one form of energy into another form
that can preferably be an electric measure. A memory 24, directly
connected to the sequencer 22, makes it possible to store calibration
data. The sequencer 22 is directly connected to a variable counter 26
that controls the switchable capacities 15 described in FIG. 1. The
variable counter 26 is typically a variable frequency divider, i.e. the
value used, for example a positive integer N, to divide a given frequency
is configurable.

[0034]The temperature sensor 21 measures an operating temperature of the
circuit at defined intervals and provides a first signal corresponding to
the measured temperature. The sequencer 22 determines a second
compensation signal integrating the value of the temperature, from the
first signal using calibration data, the second signal representing the
value N. Likewise, the sequencer 22 delivers a third signal representing
a ratio between a positive integer S and the integer N, S being less than
or equal to N. This ratio corresponds to an activity level. The variable
counter 26 is activated by the second signal and the third signal. The
counter 26 generates a fourth output signal, every N periods of a clock
signal coming from the resonator 27. This counter 26 plays the role of a
frequency divider that produces a cycle every N periods of the signal
received as input. The fourth signal corresponds to the compensated
reference frequency signal Fref. Moreover, the counter 26 has an
additional function, i.e. that of detecting the state S. The counter thus
produces a fifth signal capable of modifying the load capacity of the
oscillator 10 to produce a high or low frequency signal. This fifth
signal produces a value that changes as a function of the veracity of the
inequality defined by the state S less than or equal to the state N. The
activity level can thus be adjusted. This activity level corresponds to
the average time that the oscillator 10 spends in low frequency during N
cycles of the signal from the oscillator.

[0035]The frequency variations of the silicon-type resonator 27 as a
function of the temperature can be the subject of an approximation by a
linear-type function, the function also being able to be of a higher
order. The parameters of that function can be stored in the memory 24.
The function can be calculated by the sequencer 22 as a function of the
first signal.

[0036]The sequencer 22 also starts the operation of the temperature sensor
at defined intervals. The value of the temperature can, for example,
correspond to an electrical voltage value, itself converted into a
digital format using an analog digital converter. Using calibration data,
the sequencer 22 will therefore calculate a register value that is
applied on the variable counter 26 with the aim of changing the number of
cycles to be counted as a function of the temperature value measured by
the sensor. A register of 16 bits is typically used. When the activity
level leaves an interval between 0 and 1, the number of cycles to be
counted changes.

[0037]The linearized relative variation of the divided frequency, called
df, as a function of a temperature deviation, noted ΔT, of the
value of the division rate N and the value of the activity level, called
dc, is defined by the following equation:

[0038]where ΔfHL corresponds to the relative frequency
deviation between the high and low frequencies, the coefficient α
is the linear frequency variation coefficient of the resonator as a
function of the temperature, N0 is the division rate making it
possible to compensate the manufacturing tolerances at the calibration
temperature to obtain a given reference frequency, and dc0 is the
corresponding activity level, i.e. defined by the ratio between S and
N0.

[0039]The condition to compensate the linear drift of the silicon-type
resonator is obtained by canceling the relative variation of the
frequency. This corresponds to the following equation:

[0040]The activity level dc is then obtained as a function of the division
rate N and the temperature deviation ΔT.

[0041]The device that is the object of the invention is controlled as
follows: when the calculated value of the activity level leaves the
admissible interval, i.e. between 0 and 1, an adjustment of the division
rate is then ordered. The activity level is therefore recentered. The
sequencer 22 calculates the value N and the activity level defined by the
ratio between S and N according to equation (2).

[0042]Out of a desire to optimize the integration of the various
electronic components, one can provide for integrating, on a same
substrate in a semi-conductor material such as silicon, the sensor 21 and
the resonator 27. Indeed, the CMOS technology allows it. One can also use
two different substrates. They will then be assembled using the known
"flip-chip" method while ensuring that the resonator is under vacuum.
Although the two substrates are different, they are, preferably, formed
by the same material, typically silicon, to have the same thermal
conductivity and thereby avoid heat gradients.

[0043]FIG. 3 provides a second embodiment of a temperature compensating
device for a frequency of a silicon-type resonator. Unlike the first
embodiment shown in FIG. 2 and in which the compensated frequency is
generated directly by whole division, the second embodiment makes it
possible to generate, via a device 30, a compensated frequency that
corresponds to a power of two multiplied by the compensated frequency
obtained according to the first embodiment.

[0044]Comparably to the device 20 above, this device 30 comprises the
temperature sensor 21 connected to the sequencer 22, and the memory 24
with the same function as before.

[0045]The sequencer 22 is connected to an accumulator 31 and a dual-module
counter 32. The two elements 31 and 32 are connected to each other. The
sequencer 22 is also connected to a counting element, which can in
particular be a fixed counter 33 or an accumulator of first order or
higher order. According to the example, the counting element is a fixed
counter 33 dividing by M, whereof one of the output signals defined
hereinafter activates the switched capacities 15 of the oscillator 10.

[0046]Like the device 20, the sequencer 22 determines the second signal
representing the value N which, in the case of the device 30, is not an
integer but includes a fractional part Nfrac. A sixth signal
corresponding to the fractional part Nfrac of N activates the accumulator
31, which has the same number of bits as Nfrac. A seventh signal
corresponding to the integer part Nint of N activates the dual-module
counter 32. When it is full (carry output=1), the accumulator switches
the dual-module counter to its higher value (Nint+1) without, however,
storing (accumulating) this control bit. The accumulator 31 is typically
a discrete integrating circuit with terminals.

[0047]As in the first embodiment, the sequencer delivers the third signal
representing the ratio between S and M, S being less than or equal to M.
The fixed counter 33 is activated by the third signal. The dual-module
counter 32 generates the fourth output signal every Nint or Nint+1
periods of the clock signal coming from the resonator 27. This signal
corresponds to the compensated reference frequency signal Fref defined
above according to the diagram of FIG. 2, multiplied by 2i, i being
the number of bits of Nfrac. The result is that the average frequency of
the signal obtained over a Fref period is strictly equal to Fref.
Moreover, the counter 33 has an additional function, i.e. that of
detecting the state S less than the fixed value of the counter M. One of
the abovementioned output signals is, as in the first embodiment, the
fifth signal capable of modifying the load capacity of the oscillator 10
to produce a high or low frequency signal. This type of counter is
typically a frequency divider, which divides the value of the frequency
of an input signal by a fixed predetermined value. This fifth signal
produces a value that changes as a function of the veracity of the
inequality defined by the state S less than or equal to the state fixed
by the counter 33. The activity level between the high frequency and the
low frequency defined by the ratio between S and M is thus controlled
owing to this counter 33.

[0048]As a digital example, the first embodiment can generate a frequency
of 32 Hz from the silicon-type resonator 27 at 1 MHz. This frequency
being very specific, it is advantageous to use the second embodiment
making it possible to precisely generate the average frequency of a
quartz resonator, i.e. 32768 Hz. Indeed, given that the exact frequency
of a quartz resonator is directly equal to 210 times 32 Hz, the
division value of the counter 33 can be set at M=1024, i.e. 210.

[0049]The proposed solution is therefore compatible with any type of
device usually including a quartz and also makes it possible to generate
any compensated reference frequency Fref less than the frequency of the
silicon resonator by simple programming.

[0050]Of course, the present invention is open to various alternatives and
modifications that will appear to those skilled in the art. In
particular, it should be noted that other types of oscillators can be
associated with a silicon-type piezoelectric or electrostatic resonator.
As a non-limiting example, document EP 1 265 352 in the applicant's name
can be cited, in which a differential oscillator is disclosed falling
within the scope of the invention as defined in this application.
Moreover, a dual-frequency mode was presented in the description. The
invention would also work for a multi-frequency oscillator.